.body{

    width: 100%;
    height: 100vh;
    background-color: skyblue;
    display: flex;
    .menu{
        width: 100px;
        height: 100%;
        background-color: red;
        
    }
    .main{
        width: calc(100% - 100px);
        height: 100%;
        overflow-y: auto;
        background-color: yellow;
    }
}